Black moor swimming/floating sideways???!!!

Question
I have had my tank set up for approx 6-7 weeks.  It is 23 litre and I have gradually added fish.  I now have 2 goldfish, one black moor and 2 smaller fish (not sure what they are called - silver - one spotted and one striped).  We bought a sunken battleship for the tank and noticed that the black moor (who we have had for 5-6 wks) began spending quite a lot of time inside the ship - but all in all seemed quite happy.  I did a 1/4 to 1/3 water change on Sunday (which we have been doing weekly) and then noticed on Monday evening, the black moor was at the top of the tank floating on its side.  He stayed there for a little and then seemed to shake it off and started swimming around the tank as though there was nothing wrong.  We fed them and went to bed.  This morning (Tues) I checked on him and he was doing the same - although his swimming didn't seem to be as good - a little wonky. We have a underground filter which covers most of the bottom of the tank, and a smaller sponge type filter in the opposite corner.  We noticed this morning that the sponge filter was not working properly and have since cleaned it out and now have it working again.  We had the water tested by the pet shop only a couple of weeks ago and it was fine then.  I have only been feeding them fish flakes and have only just realised that they also should be fed things like cooked peeled peas, beans etc...  Is my fish going to be okay?  All the others seem to be fine. What should I do.  I have read a little about swim bladder problems and he does seem to float vertically at times too.  Please help me try to save him!!

Answer
Hi Lyndall;

It sounds like a digestive problem that is affecting his swim bladder. Here is a link to information about it and possible treatments;

http://www.petplace.com/fish/when-your-fish-can-t-swim/page1.aspx#

Your fish really do have another serious issue facing them soon too. The swim bladder failure could be related to it. The tank is way overcrowded for the types of fish you have in there. Goldfish need about 40 liters per fish as they grow to their adult size. They may be small now but they will grow and will grow quite fast. The moor is a fancy type of goldfish and they all get to be 20 cm long or larger, not including their tails. In a few weeks you will need to get them at least a 120 liter tank. Very soon, their own waste will suffocate them and they won't be able to move very well due to their size in such a small place. Poor guys. Here is more information about goldfish and their needs;

http://www.firsttankguide.net/goldfish.php

Until you can get them a bigger tank, replace 25% of the water 3 times a week. It will hopefully keep them alive until the new tank is ready.

I hope all goes well.......

At Your Service;
Chris Robbins